Photographic processes for obtaining direct positive images without employing a reversal processing step or a negative film are well known.
Taking practical utility into consideration, conventional techniques for obtaining a positive image from a direct positive silver halide photographic light-sensitive material, exclusive of certain special methods, are divided chiefly into the following two types.
One type employs a previously fogged silver halide emulsion whose fog centers (latent image) in exposed areas are destroyed making use of the solarization or Herschell effect to obtain a direct positive, after development.
The other type uses an internal latent image type silver halide emulsion not having been fogged, which is imagewise exposed to light and then subjected to surface development either after fogging treatment or during fogging treatment to obtain a direct positive. The internal latent image type silver halide emulsion used herein is such an emulsion in which silver halide grains have sensitivity specks predominantly in the interior thereof and form a latent image predominantly in the interior upon exposure to light.
The methods belonging to the latter type generally enjoy higher sensitivity and are suitable for uses requiring high sensitivity as compared with the methods of the former type. The present invention belongs to the latter type.
Various techniques of this type have been known, such as those disclosed, for example, in U.S. Pat. Nos. 2,592,250, 2,466,957, 2,497,875, 2,588,982, 3,317,322, 3,761,266, 3,761,276 and 3,796,577, and British Patent Nos. 1,151,363, 1,150,553 and 1,011,062. According to these conventional techniques, photographic materials providing a direct positive image with relatively high sensitivity can be produced.
For the details of the direct positive image formation mechanism, reference can be made to, for example, T.H. James, The Theory of the Photographic Process, 4th Ed. (1979), Chap. 7, pp. 182 to 193 and U.S. Pat. No. 3,761,276.
It is believed that a direct positive image is formed through the following mechanism: First, imagewise exposure results in the formation of an internal latent image (positive hole) in the interior of silver halide grains, which leads to the formation of fog centers selectively on the surface of the unexposed silver halide grains by surface desensitization based on the internal latent image, and subsequent conventional surface development processing results in formation of a photographic image (a direct positive image) on the unexposed area.
Selective formation of fog centers can be generally effected by a "light fogging method" in which the entire surface of a light-sensitive layer is secondarily exposed to light as described, for example, in British Patent No. 1,151,363 or a "chemical fogging method" using a nucleating agent described, for example, in Research Disclosure, Vol. 151, No. 15162 (November, 1976), pp. 76 to 78.
In the formation of a direct positive color image, the internal latent image type silver halide light-sensitive material is subjected to surface color development processing either after or simultaneously with fogging treatment and then subjected to bleaching and fixing (or bleach-fixing), After the bleaching and fixing processing, the material is usually washed with water and/or subjected to stabilizing processing.
In the chemical fogging method hitherto used, nucleating agents which exhibit their effective nucleating function only at a high pH range of 12 or higher are employed. Under such a high pH condition, the developing agent is more susceptible to deterioration due to air oxidation, so that development activity becomes seriously reduced. Further, the rate of development is low and thus a long processing time is required. Particularly, in case of using a developing solution of a low pH, the processing time becomes much long.
On the other hand, the light fogging method does not require a high pH condition and, therefore, enjoys a relatively practical advantage. However, this method encounters various technical problems when applied to a broad photographic field for various purposes. More specifically, since this method is based on the formation of fog centers by photolysis of silver halide, the optimum illumination or exposure varies depending on the kind and characteristics of the silver halide used. It is, therefore, difficult to assure predictable performance. In addition the development apparatus required is complicated and expensive.
Hydrazine compounds are well known as nucleating agents which are employed in the above-described chemical fogging method In general, hydrazine type nucleating agents are excellent in view of discrimination since they provide a large difference between maximum density (Dmax) and minimum density (Dmin). However, they are disadvantageous because they require processing at a high pH (pH &gt;12).
As nucleating agents which function in processing at a low pH (pH .ltoreq.12), heterocyclic quaternary ammonium salts are known and described, for example, in U.S. Pat. Nos. 3,615,615, 3,719,494, 3,734,738, 3,759,901, 3,854,956, 4,094,683 and 4,306,016, British Patent No. 1,283,835, JP-A-52-3426 and JP-A-52-69613 (the term "JP-A" as used herein refers to a "published unexamined Japanese patent application"). Particularly, propargylor butynyl-substituted heterocyclic quaternary ammonium salt compounds described in U.S. Pat. No. 4,115,122 are excellent nucleating agents in view of discrimination when used in direct positive silver halide emulsions. However, in silver halide emulsions wherein sensitizing dyes are employed for the purpose of spectral sensitization, competitive adsorption of the sensitizing dyes and the heterocyclic quaternary ammonium type nucleating agents onto silver halide grains takes place, and thus, it is necessary to add a large amount of the quaternary ammonium salt type nucleating agents which are of low adsorptivity. In particular, in the case of multilayer color photographic light-sensitive materials, unevenness of density and destruction of color balance may undesirably occur. Therefore, these compounds are still insufficient. The above-described problems tend to become remarkable during storage under high temperature and high humidity conditions.
In order to resolve the above-described problem, quaternary salt type nucleating agents having an AgX adsorption accelerating thioamido group are disclosed in U.S. Pat. No. 4,471,044. Although the amount to be added necessary to obtain a sufficiently high Dmax is reduced and the decrease in Dmax during storage at high temperature is controlled by introduction of the adsorptive group, these effects still do not achieve to a fully satisfactory level.